The ideal laying of a submarine cable is to place a cable along the sea bottom with minimum necessary length of cable in order that a submarine cable may be laid with just sufficient slack. Therefore, a conventional cable ship is equipped with a drum type or a linear type cable engine.
In a cable laying operation between two points, a cable engine is able to pay out a cable by length which is actual distance between two points and some slack which is additional length of several % of the distance. The slack is determined from topography data about the sea bed which is preliminarily surveyed, and the speed of a cable ship across the sea bottom measured by using a taut wire gear.
On the other hand, in cable burial operation, a cable paid out must pass through a cable burier before being set on the sea bed. Even if a cable engine pays out a slack, the slack will be stocked or stored at the entrance of the cable burier before the slack reaches the sea bottom, and the cable would bend, and/or would be rubbed and damaged.
Therefore, for safety so that a cable is not damaged, a cable is paid out with high tension, so as not to be stored at the entrance of a cable burier. As a result, a paid out cable would be unburied because of high tension, even if a trench is dug in the sea bed with enough depth. For that reason, a prior drum type cable engine and a prior linear type cable engine are not suitable for burying a cable.
Recently, an iron ring cable guiding equipment or a cylindrical cage (JP patent laid open 7010/1991) is used for providing a path of a cable and repeaters from a ship to a cable burier. That equipment or a cage is also useful for realizing ideal "no-cable-tension, and no-cable-slack" condition to a burial cable.
The iron guiding equipment, however, has the disadvantage that it is heavy in weight and it needs wide space for storage. Therefore, the burial operation by using an iron guiding equipment is limited up to 200 m in sea depth. Accordingly, it does not meet the latest demand of burial operation in the deeper (over 1000 m) sea bed.
The cylindrical cage which is made of cloth has no such disadvantage, and it discharges well its duty as a cable guiding equipment in deep sea water. However, when it is used as a cable engine in deep sea water, because of the power of brake standing for the factor Wh, which is mentioned later, it is impossible to compare favorably with a foresaid drum type cable engine etc.
Therefore, at present, no cable engine for a burial operation in deep sea exists.